Chemical and electro-chemical extraction of chromium from its ores



Aug. 20, 1957 G. c. WESTB Y 2,803,594

CHEMICAL AND ELECTRO-CHEMICAL EXTRACTIONOF CHROMiUM FROM ITS ORES Filed March 23. 1951 2 Sheets-Sheet 1 CHROM ITE ORE Roila acid a are 4-\ mend sinengih. 76 5 Q 50C.

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OHROMIUM M METAL Gttomegss United States Patent CHEMICAL AND ELECTRO-CHEMICAL EXTRAC- TION OF CHROMIUM FROM ITS ORES George C. Westby, Sacramento, Calif., assignor to Key Metals Corporation, Seattle, Wasln, a corporation of Washington Application March 23, 1951, Serial No. 217,188

1 Claim. (Cl. 204-105) This present invention provides a method of extracting chromium from chromite ore and includes the use of chemical means to assist electrolytic action in that process for economy in operation. The process is especially directed to the recovery of chromium from the black chromite sands, in disseminated chromite deposits, and in general substandard chromium deposits, occurring in nature at various points throughout the world. In the past the methods of electrolysis have failed in the com mercial recovery of chromium from these black sands, due firstly to an insufiicient percentage of recovery, and secondly to the impure form of that chromium which necessitated further and expensive treatment to obtain sufficient purity for commercial uses. Following the present method, it has been found that, by the chemical and electrolytic combination used, chromium metal of very high puritycan be produced from even low grade ores with very high current efiiciency and at quite low cost.

The principal object of this invention is to provide a highly eflicient commercial system for the extraction of chromium metal from chromite ores, specifically from such deposits as certain black sands wherein both chromite and ilmenite and/or rutile occur in a disseminated form, by the use of reagents which can be systematically regenerated and automatically controlled by alternations of oxidation and reduction of chromium solutions motivated primarily 'by the oxygen and oxygen carriers of the cell and the sulphur dioxide used in regeneration of the sulphuric acid used in the electro leaching system. A further object is the production of electrolytic chromium which is obtained with maximum efficiency by first reducing the chromic sulphate to chromous sulphate preferably by chemical means, then reducing the chromous sulphate to the metal electrolytically.

A further object is the conservation of anodic oxygen by utilizing chromic sulphate in the anodic compartment of the cell which is readily oxidized to the hexavalent form which is then available as an effective reagent for forming sulphuric acid from sulphur dioxidesuch as that derived from burning sulphur in air.

A further object of this present invention is to protide a method for the recovery of chromium from its ores that is a simple continuous flow process using but few reagents and these of low cost.

A further object of this invention is to provide a method of recovering chromium wherein the multiplicity of process steps, apparatus, crystallization tanks, evaporators, digesters, furnaces, coolers, centrifuges and filters as used in present day hydro-metallurgy of chromium is avoided.

catholyte and transfer of the S04 ions to the anolyte,

and by full utilization of atmospheric oxygen and the ,oxidized chromic sulphate and other anodic oxidized compounds as iodic and titanic acids.

A further object of this invention is to provide a method in which the continuous circulation of titanous sulphate through the catholyte is used eifectively and economically as a method of reducing chromic sulphate and preventing oxidation of the catholyte and thus improving current efficiency.

A further object is to provide for the simple separation of chromium compounds from other materials in the chromite ore.

Another object is to minimize costs of operation by minimizing the use of expensive agents which are not by-products of the process and would represent a continual added cost of operation, and also to minimize the bulk and weight of materials that must be transported to the site of operation.

Further objects, advantages and capabilities will be apparent from the description and disclosure in the drawings, or may be comprehended or are inherent in the process.

The drawings, Figure 1 and Figure 2 are schematic flow sheets of specific examples, Examples I and II respectively, of a chemical and electrolytic method employed in the utilization of this present invention.

The principal product of the system of ore treatment is the metal chromium and this invention is especially devised to manufacture chromium metal of over 99.7% purity from low grade or disseminated ores at a cost low enough to compete successfully with other known processes.

The ore on which much of applicants experiments was centered pertained to a concentrate of a black sands deposit situated north of Bandon, Oregon. The concentrates assayed:

Percent C1'203 43.3 FeO 26.5 TiOz 0.8 MgO 22.8 CaO 1.3 A1203 1.0 SiOz 3.4

The ore of analysis given was tested under the appended conditions:

Through 5O mesh-concentration of acid-% H2804.

Temp. 225 degrees C.-ratio of acid to ore 4 to 1.

Test showed 94% chromium extracted in the form of anhydrous CR2(SO4)3.

A sample of the same ore ground to pass 200 mesh with the same general conditions gave complete extraction.

contemporaneous tests showed that sulphur burnt to S02 will form H2804 free of thionic acids as H2520 HzSsOs etc., when in the presence of H2804.

Due to economic conditions the utilization of the simplest and cheapest reagents is imperative in treating most such deposits. Experiments indicate that. sulphur is the best possible substance to use for this purpose since its weight for equivalent extraction is /3 of H2804, whereby a minimum of weight need be transported to sites of operation of difficult access; its cost per unit of sulphur about /3 of H2804; and because sulphur derives when burnt, another necessary reagentoxygen from the air and also gives off much available energy in the form of heat.

Referring to the fiow sheet, Example I attached to this specification shows a system developed into three inter-locking processes-annotated A, B and C where A represents the cathodic circuit, B pertains to the anolyte circuit of the electrolytic process and C is 'the leaching circuit.

A consists of a closed catholyte circuit including a eatholyte compartment separated by a ceramic diaphragm Patented Aug. 20, 1957 from the anolyte (and anode) and a closed top which retains a hydrogen atmosphere in the system. The cathode may be steel or lead. In practice the catholyte of circuit A flows slowly but continuously through the closed circuit, preferably including a regulating tank where material from the leaching system is first introduced and conditioned as a standardized catholyte.

B consists of the anodic circuit of the electrolytic cell and the anolyte flows slowly but continuously through this circuit. Circuit B includes, preferably, a tank where sulphur dioxide is introduced in the production of sulphuric acid in the anolyte system. As indicated in the flow sheet, sulphuric acid produced in the anolyte is directed into circuit C for leaching of further chromite ore. The anode is preferably lead, on which a film of lead peroxide is formed beneath the surface of the anolyte. The lead peroxide accelerates the oxidation of the oxidizable substances or oxygen carriers in the anolyte.

Circuit C pertains to the leaching process covering dissolution of the chromite ore with the formation of chromium and other metallic constituents as sulphates. As was noted above, part of the leaching process is carried out by sulphuric acid generated in the anolyte and sometimes other materials from the anolyte are also used as oxidizing agents.

With black sand of the Bandon type, aforementioned, the separation of the chromium from other materials is conducted as follows and as illustrated on the accompanying flow sheet:

Sulphuric acid, of a strength equal to the strength of 76 to 80 percent, at a temperature of 150 degrees centigrade, is added to the chromite ore with approximately the ratio of ore to 100% acid of l to 4. If there are considerable insoluble silicious materials and calcium sulphate formed these are separated from the soluble sulphates by filtration. The solution of sulphates, containing sulphuric acid, is heated to between 170 and 225 degrees centigrade until the anhydrates of the metallic sulphates are formed. Following formation of the insoluble anhydrates the solution is allowed to settle and the acid is decanted to be used in the treatment of further ore. Then water is added and the water soluble anhydrate are taken in solution and treated as by-products. It will be noted that these soluble materials include titanous sulphate which is separated from the other materials and used later on in the process.

The insoluble anhydrates, in the case of the Bandon chromite ore, consist usually of the anhydrates of chromic and ferric sulphates. The addition of water and certain agents of the proper type cause the ferric sulphate to become soluble and it is drained 01f. These agents include ferrous sulphate. Another agent which may be used is 90% sulphuric acid, the ferric sulphate becoming soluble while the chromic sulphate remains insoluble.

At this point water is added and a particle of activated chromium is added to the wet, slightly acid anhydrous chromic sulphate; and the chromic sulphate dissolves and is added to the catholyte in circuit A.

This activated chromium can be taken from chromium metal freshly formed in the catholyte compartment of the cell. The anhydrous chromic sulphate ordinarily will not go into solution without the presence of the activated chromium. The step of adding water is preferred but it may be eliminated by adding the anhydrous chromic sulphate directly to the catholyte system. The freshly formed chromous sulphate in the catholyte will insure that the anhydrous chromic sulphate will go into solution. Some of this chromic sulphate may be added to the anolyte to be there oxidized and then reduced in the formation of sulphuric acid, as will be explained later.

One of the important features of my process is the use .of titanium in the reduction of the chromic sulphate.

Through the use of titanous sulphate for that reduction, as illustrated in the appended flow sheets, the-re is a saving of one third of what otherwise would be the amount of electricity necessary to plate out the chromium in the electrolytic cell. As was noted above, titanous sulphate is a by-product of the separation of the various materials in the ore, so there is no extra cost involved in the use of this material. If there were not this titanium content in the ore or were it not present in sufiicient quantity, titanium could be furnished quite cheaply for this reduction in the form of material such as ilmenite. Without the use of the titanium the cost of the operation would increase materially. Other materials to obtain a similar action generally are more expensive. Before the conception of the present process, titanium had not been used in the production of chromium from chromite ore.

When the titanous sulphate is added to the chromic sulphate a reduction of the latter is obtained, as: Tl2(SO4)3 plus Cr2(SO-;)3 equals 2CrSO4 plus 2Ti(SO4)z. Sufficient titanous sulphate is added to the catholyte system to reduce all of the chromic sulphate to chromous sulphate. Gne of the important features of my process, which is common to many materials in the various circuits and passing between those circuits, is that the variou agents used are often changed back to their original form and used time after time instead of becoming waste material. This feature is true of the titanous sulphate. After being changed to titanic sulphate it is reduced by various means to titanous sulphate and is then reused to reduce further chromic sulphate. The titanic sulphate is reduced by means of the hydrogen atmosphere maintained in the catholyte system or by other means as in the use of regenerated hydroiodic acid in the electrolytic cell or by cathodic nascent hydrogen. Under some conditions, sulphur dioxide could also be used to reduce the titanic sulphate.

By electrolytic action, the S04 ion is separated from the chromous sulphate and the metal chromium is plated out in the cathode compartment of the electrolytic cell. The S04 ion migrates to the anode compartment of the cell. To prevent oxidation of the chromou sulphate, a hydrogen atmosphere is maintained in the catholyte system and the pH is maintained at a level, preferably between 2.2 and 2.4 and the limits of 1.6 and 2.6 are critical. Other inert gases could be formed in the process instead of hydrogen, to prevent oxidation of materials therein. Hydrogen or inert gases could be termed non-oxidizing, both having the purpose of preventing oxidation of chromous sulphate formed. Hydrogen is normally present in the catholyte during the electrolytic deposition of chromium metal and aids in the maintenance of titanous sulphate. Mannitol, carbonates or other oxidation-retarding agents can be added as another method of preventing oxidation of the chromous sulphate.

Circuit B contains a fixed amount of chromium. The chromium is usually added in the form of chromic sulphate. This is alternately oxidized to a chromate and then reduced back to chromic sulphate in the anolyte system. The oxidation and reduction of the chromium in the anolyte is another example of the repeated use of an agent in my process, instead of treating the material to obtain only one result and then discarding the same as waste material. Prior to the conception of the present process chromic sulphate has not been used in the anolyte in the production of chromium from chromite ore and this conception is one of the important contributions of the described process. The chromium used in the anolyte of course is obtained as a product of the process and does not add to the cost of the process as an expensive agent to be purchased. Agents that must be separately purchased add materially to the cost of the operation even though it is to be used. over and over again, because of losses incident to the handling'of the materials.

The chromic sulphate is oxidized in the anolyte, as: Cr2(SO4)3 plus 5H2O plus 03 equals 3H2SO4 plus 2HzCrO4 (in the presence of the lead peroxide coating on the anode). The chromate is reduced, preferably in the generating tank of the anolyte system, by the addition of asoaeoi sulphur dioxide, as a direct reaction or a reaction with sulphurous and sulphuric acids formed by the sulphur dioxide. Thus chromic sulphate is again formed and the alternating oxidation and reduction repeats.

The purpose of the chromic sulphate-chromate cycle, is to retain oxygen formed in the anolyte until it can be combined with sulphur dioxide to form, ultimately, sulphuric acid. The sulphuric acid is directed back to the leaching system to process further chromite ore, as shown in the attached flow sheet, Example I. The use in the anolyte system, in the electrolytic extraction of chromium from chromite ore, of chromic sulphate for the purposeful production of sulphuric acid to be used in the original leaching of the chromite ore is a new feature of my invention. Further, the method is new of producing sulphuric acid in the anolyte by use of a material such as a chromate which retains oxygen produced in the anode circuit and adding sulphur dioxide to such material in order to reduce the material and form sulphuric acid. The reaction, upon addition of sulphur dioxide, may be first acombination with water to form sulphurous acid and then the reaction with the chromate and sulphurous acid. An important feature of this process is the idea of adding sulphur dioxide to the anolyte. The sulphur dioxide is produced by the burning of sulphur and it will be apparent that this method is much improved over that of purchasing and transporting sulphuric acid to the site of operations. The site of operations often is difiicult of access and it is much easier to transport the sulphur than the sulphuric acid. Furthermore, the sulphur involves but a fraction of the cost of prepared sulphuric acid. The sulphur is oxidized in the presence of sulphuric acid in order to avoid the formation of a thionic acid.

The oxygen is generated in the anolyte in various ways. For instance, the S04 ions, migrating from the catholyte to the anode upon the separation of chromium metal from chromous sulphate, combine with water to form sulphuric acid releasing oxygen. There is a certain amount of direct electrolysis of the water producing oxygen on the anode side of the cell.

A variation of the method, especially in the use of a material to combine with oxygen in the anolyte for later reaction with sulphurous acid formed by the sulphur dioxide, is the use of hydroiodic acid. The hydroiodic acid may be added to the catholyte. Due to electrolytic action, the acid may break up into hydrogen and iodine ions migrating to the anode side of the cell. Another reaction might be: Cr2(SO4)3 plus 2H1 equals 2CrSO4 plus I2 plus H2804, the iodine again migrating to the anolyte. Various reactions may take place in the anolyte, as the formation of iodic acid, i. e.: 50 plus I2 plus H2O equals ZHIOs. Periodic acid might likewise be formed. With the iodic or periodic acid formation, the action of the iodine compounds is similar to that of the chromates. That is, oxygen and hydrogen are combined with the iodine to form such acid, as in H103 and H104; and then the acid reacts with sulphur dioxide or sulphurous acid to ultimately generate sulphuric acid. Although iodine has been used before in the recovery of chromium by electrolysis, it has not been used in the anolyte to combine with oxygen and then sulphur dioxide or its products to form sulphuric acid. The various means used to conserve the oxygen formed in the anolyte for the formation of sulphuric acid are important as realizing a very considerable savings in the recovery of chromium.

Other chromates, and dichromates, could be used in the anolyte instead of the H2CX'O4 formed by the addition of chromic sulphate, i. e. K2CI'04, NazCrOt, MgCrOa, etc.

Another variation of the example of the process Example I is the use of titanium instead of chromium in the anolyte circuit B. The titanium would be introduced in the form of titanous sulphate. The oxygen generated in the anolyte would oxidize the titanous sulphate to TiO(SO4)2. The TiO(SO4)2 later reacts with S02 or H2803 to form titanous sulphate and the circuit repeats, the oxygen and sulphur dioxide resulting in the production of sulfuric acid to be directed into circuit C for the leaching of further chromite ore.

Additional oxidation-retardants can be added to the catholyte to prevent oxidation of chromous sulphate therein or even to reduce chromic sulphate to chromous sulphate, as for instance mannitol, titanous sulphate, etc.

An important addition to the cathode compartment of the cell is scrap chromium metal. I have previously used the scrap chromium as an additive for obtaining high current efficiencies but the use of chromium scrap as a stabilizer is new. This is a stabilizer of the catholyte. For instance, the scrap chromium will react with chromic sulphate as: Cr plus Cr2(SO4)3 equals 3CrSO4. Thus the scrap chromium prevents the oxidation of chromous sulphate to chromic sulphate. It will be noted that when the scrap chromium reacts to form chromous sulphate then the scrap chromium will ultimately be deposited in the cathode compartment in the form of pure chromium metal. As the scrap chromium is often not suitable for many uses because of the impurities therein, the scrap sells at a very low price and there is considerable economic advantage in processing the same to the form of pure chromium metal. Although it is preferred in some operations to reduce the-chromic sulphate with titanous sulphate, other operations close to a supply of the scrap chromium may wish to use it as a primary reducing agent. With the use of sufiicient titanous sulphate, the scrap chromium remains inert below the cathode under a covering of chromous sulphate.

Example I is the preferred flow of materials in my process. However, with some ores some objectionable material cannot readily be separated from the chromium content by the flow in Example I, as for instance when there is vanadium present; and the flow shown in Example 11 may preferably be followed in the extraction. Referring to the flow of Example II, the leaching step may be conducted in the same manner as Example I. To the residue enough water may be added to produce a pH of approximately 2.0 in the solution. This material then runs through the anolyte compartment of the electrolytic cell. The chromium content combines with oxygen therein and changes to a chromate. This is similar to the action in circuit B of Example I in changing chromic sulphate to a chromate.

A dilute solution of the materials is then heated to the boiling point. Following the boiling hydrolyzed Ti(OH)3 is removed as by filtering. The iron and aluminum content is then separated by the addition of alkalizing matter such as alkaline earths, i. e., MgO and CaO. The chromate then may be separated as by filtering. The chromate is approximately neutral and in adding paperwaste thereto, enough sulphuric acid or sulphur dioxide is added to acidify the solution to approximately 2.0. The chromate and paperwaste is heated and carbon dioxide gas is produced. The chromic sulphate formed and other material present is then added to the catholyte tank. The chromic sulphate, as in Example I follows circuit A and chromium metal is formed.

Various modifications of the process will be apparent to those skilled in the art. In working with various ores certain well known adjustments may be necessary as within the skill of the art. In Examples I and II there may be certain materials present in different ores which are not indicated or some material may be left out, depending upon the type of ore being processed. It is felt that the materials shown, such as iron, aluminum, sodium, etc. are typical. The sulphuric acid and water content is not always indicated in the boxes and it will be apparent when they are present.

Dealing with some chromite ores, it is necessary to have a source of oxygen, in addition to the sulphuric acid, to change the metallic oxides into sulphates, for instance: 2Cr2OaFeO plus 9H2SO4 plus 0 equals 2Cr2(SO4)3 plus Fe2(SO4)3 plus 9H2O; or 2Fe0 plus 3H2SO4 plus 0 equals Fe2(SO4)3 plus 31-120. This oxygen may be furnished in the form of a chromate returned from the anolyte system or may be furnished in other forms, as in the form of titanic sulphate.

In general applicant would draw attention to the following notes regarding the system as a whole:

(a) Simplicity of operation on a continuous flow basis.

(b) Low cost of the few reagents used.

(0) Low cost of sulphuric acid regeneration and production from sulphur and its direct development from the oxidized chromic sulphate by anodic action.

(0.) The fact that by-products of commercial consequence are cheaply recoverable in the .form of sulphates, indicated on the attached flow sheet.

(e) The simple leaching process separation of chromium from other elements.

(f) That motivation of the system is controlled by alternation of reduction and oxidation which is based primarily on the electrolytic chromium deposition in the cathode compartment of the cell, and secondarily on the quantity of oxygen released at the anode.

(g) That the method of maintenance in a closed system of easily reducible oxygen as chromates is the most efiiective way to store and to use oxygen.

(h) That the charge of chromic sulphate to the anolyte assures continuous absorption of the free anodic oxygen given ofi from the anode.

(i) That the continuous circulation of titanous sulphate through the catholyte compartment is an effective and economic method of preventing oxidation of the catholyte and improving current efficiency.

(j) That the multiplicity of the process steps, apparatus, crystallization tanks, evaporators, digesters, furnaces, centrifuges and filters used in present day hydro-metallurgy of chromium is avoided.

Having thus described my invention, I claim:

The process comprising aqueously reducing trivalent chromic sulfate with titanous sulfate whereby chromous sulfate is formed, and electrolyzing the latter to produce metallic chromium.

References Cited in the file of this patent UNITED STATES PATENTS Lloyd May 9, 1950 Westby Dec. 11, 1951 Transactions of the Electrochemical Society, vol. 89 (1946), pp. 443453 (an article by Lloyd et al.). 

